This disclosure related to fabrication of thin film photovoltaic cells. Solar cells are electrical devices for generation of electrical current from sunlight by the photovoltaic (PV) effect. Thin film solar cells have one or more layers of thin films of PV materials deposited on a substrate. The film thickness of the PV materials can be on the order of nanometers or micrometers.
Absorber layers absorb light for conversion into electrical current. Examples of thin film PV materials used as absorber layers in solar cells include copper indium gallium selenide (CIGS) and cadmium telluride. Solar cells also include front and back contact layers to assist in light trapping and photo-current extraction and to provide electrical contacts for the solar cell. The front contact typically comprises a transparent conductive oxide (TCO) layer. The TCO layer transmits light through to the absorber layer and conducts current in the plane of the TCO layer. Some solar cells include a buffer layer between the absorber layer and the TCO layer. The buffer layer can increase cell efficiency, increase open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF), compared to a similar solar cell without a buffer layer.
Suitable buffer layers, such as CdS and ZnS, have been used in high efficiency CIGS solar cells. The buffer layers function as n-type materials to form a p/n junction with p-type CIGS or CdTe. Typically, CdS and ZnS-based buffer layers are prepared using chemical bath deposition (CBD) due to its low cost, simple processing and scalability.